As is well-known in the art, an internal combustion engine mixes air with fuel, and ignites and burns the mixture of air and fuel in a controlled manner to produce work. The resulting exhaust gases, which may be treated to remove pollutants, are then expelled into the atmosphere. Ignition of the air/fuel mixture in the cylinder is typically achieved by an ignition device, typically, a spark plug or the like, or by the adiabatic compression of the air/fuel mixture, which heats the mixture to a temperature above its ignition point.
In gasoline powered internal combustion engines commonly in use today, ambient air is conveyed via an air intake duct or port to a carburetor or a fuel injection system, which is used to mix the air with the fuel to create the air/fuel mixture. For engines with some types of fuel injection systems, as well as those equipped with carburetors, the air/fuel mixture is then conveyed via an intake manifold to the combustion chamber or cylinder of the engine. In gasoline engines equipped with port injection type fuel injection systems, the air is directed through the intake manifold to the intake port of the combustion chamber before the fuel is mixed with the air. In diesel-type engines and some gasoline engines using in-cylinder fuel-injection systems, the air and fuel are conveyed separately to the combustion chamber or cylinder of the engine where they are mixed.
After the combustion of the air/fuel mixture, the resulting exhaust gases are expelled from the combustion chamber to an exhaust manifold. In almost all modem gasoline powered engines, the exhaust gases are then conveyed by an exhaust pipe to a catalytic converter where pollutants are substantially removed from the exhaust gas. However, during the operation of an internal combustion engine, even one equipped with pollution control devices, such as a catalytic converter, some pollutants, as described below, remain in the exhaust stream, and are expelled into the atmosphere through a tailpipe.
In addition to complete combustion products, such as carbon dioxide (CO.sub.2) and water (H.sub.2 O), internal combustion engines also produce exhaust gases containing a number of pollutants, e.g., carbon monoxide (CO), a direct poison to human life, and hydrocarbons (HC), that result from incomplete combustion. Also, due to the very high temperatures produced by the burning of the hydrocarbon fuels followed by rapid cooling, thermal fixation of nitrogen in the air results in the detrimental formation of nitrogen oxides (NO.sub.x), an additional pollutant. The amount of CO, HC, NO.sub.x and other pollutants produced by an internal combustion engine varies with the design and operating conditions of the engine. Other pollutants that can be produced by a combustion engine include particulates, which may include solid carbon and, possibly, various heavy hydrocarbons.
Although the presence of pollutants in the exhaust gases of internal combustion engines has been recognized since 1901, the control of internal combustion engine emissions in the United States only became required by law with the passage of the 1970 Clean Air Act. Engine manufacturers have explored a wide variety of technologies to meet the requirements of this Act, including exhaust gas recirculation, electronically controlled fuel injection systems, which receive data from various sensors in the combustion stream, allowing the accurate control of the air/fuel ratio, and catalytic converters. Catalysis has proven to be the most effective passive system.
The purpose of a catalytic converter is to oxidize certain pollutants, such as CO and HC to CO.sub.2 and H.sub.2 O, and, in a three way catalyst, to additionally reduce NO/NO.sub.2 to N.sub.2. In modern three way catalytic converters (TWC), all three pollutants are reduced simultaneously. NO.sub.x reduction is most effective in the absence of oxygen, while the abatement of CO and HC requires oxygen. Therefore, to prevent the production of these emissions from presently available vehicles requires the operation of the engine at or near the stoichiometric air-to-fuel ratio.
Today, nearly all automobile catalytic converters contain noble metal catalysts, held in honeycomb monolithic structures, which have excellent strength and crack-resistance under thermal shock. The honeycomb construction and the geometries chosen provide a relatively low pressure drop and a large total surface area that enhances the mass transfer controlled reactions that remove pollutants from the exhaust. The honeycomb is set in a steel container, and protected from vibration by a resilient matting.
An adherent washcoat, generally made of stabilized gamma alumina into which the catalytic components are incorporated, is deposited on the walls of the honeycomb. TWC technology for simultaneously converting all three pollutants typically utilizes the precious or noble metals platinum (Pt) and rhodium (Rh), where Rh is primarily responsible for the reduction of NO.sub.x, while also contributing to CO oxidation, and Pt is primarily responsible for CO oxidation. Recently palladium, Pd, has been substituted for or used in combination with Pt and Rh.
While considerable gains have been made in recent years to reduce the pollutants emitted in the exhaust gases from automobile and truck engines, further reductions in the amount of pollutants in the exhaust gases of the internal combustion are required under present and planned government regulations.
Increasing the efficiency of the catalytic converter or catalysis is highly desirable. The conversion efficiency of a catalytic converter is measured by the ratio of the rate of mass removal of the particular constituent of interest to the mass flow rate of that constituent into the catalytic converter. The conversion efficiency of a catalytic converter is a function of many parameters including aging, temperature, stoichiometry, the presence of any catalyst poisons, such as lead, sulfur, carbon and phosphorous, the type of catalyst, and the amount of time the exhaust gases reside in the catalytic converter.
Prior art attempts to increase the efficiency of catalytic converters have not been sufficiently successful. Modern catalytic converters significantly reduce CO, HC, and NO.sub.x, but use expensive noble metal catalysts, and may have difficulty in meeting future emission requirements. Moreover, commercially available catalytic converters have a limited performance lifetime, and have a low conversion efficiency until the catalyst reaches operating temperature.
Therefore, a need exists for a simple, inexpensive means of reducing the amount of pollution released into the environment from gas streams, such as the gases produced by the combustion of fuel in an engine, in a fuel cell reformer, in effluent gas from manufacturing processes, etc., that can be installed on systems presently in use that release pollution, as well as on newly manufactured systems. The present invention provides such a means.